This invention pertains to a new class of multiplexing filter structures, which filter structures have both the properties of channelized filtering and channelized linear frequency discrimination. Specifically, the channelized filter-discriminator of this invention is intended for use as part of a microwave receiving system possessing a high probability of intercept for incoming signals. The selective properties of the band pass filters provide an interference-reduction capability while the discriminator property provides for instantaneous frequency measurement. In particular, the design procedure for achieving a very high degree of discriminator linearity in association with a reasonable selectivity comparable to a maximally flat filter uses theoretical design data provided in a normalized table generated by computer optimization.
A number of methods for building multiplexers with standard responses (not directed at frequency discrimination) are in current use (J. D. Rhodes and R. Levy, "Design of General Manifold Multiplexers", IEEE Transactions on MTT, Vol. MTT-27, pp 111-123, February, 1979.) The one chosen here for the discriminator application is the terminated-manifold method (W. A. Edson and J. Wakabayashi, "Input Manifolds for Microwave Channelizing Filters", IEEE Transactions on MTT, Vol. MTT-18 pp 270-276, May, 1970) because it is the most suitable for a large number of relatively narrow channels. The even and odd-numbered channels appear on separate manifolds isolated by a power divider and by isolators. The individual channel bandpass filters are coupled to the manifold through transformers that allow a percentage of the power to be coupled into a particular filter if the signal frequency is in its passband. The remaining power is absorbed in the matched load terminating the manifold. At frequencies outside its passband each filter becomes decoupled from the manifold because the input resonator is equivalently series. Since filter frequencies are separated by at least one channel width, very little interaction occurs along the manifold. Interaction between adjacent channels (on opposite manifolds) is negligible because of the isolators.
Each filter has a bandpass response but in addition the response shape is such that the ratio of the output levels of two adjacent channels (their difference in dB) follows a straight-line law for dB vs frequency in the frequency range between the two adjacent channel centers. In a system having many channels, each channel works with its higher and lower-frequency neighbors to produce this same result. A logarithmic detector is used on each filter output.
Early analysis (C. K. Clark, "A High-Probability-Of-Intercept ESM Receiving System For Dense Signal Environments", GTE Report PRE-1143, February 1983) showed that a linear discriminator characteristic using the process described above could be produced by filters with perfectly Gaussian shapes. Rather than trying to synthesize such filters directly, the present invention has as its principal object a computer optimization technique to force the dB difference of two adjacent filter outputs to be a prescribed straight-line law (vs frequency) within a prescribed equal-ripple error.